Why does this cast NOT fail?

Question

Was testing the behavior of casting, lists, etc. and ran into something that I can not quite figure out. When casting a list into a list that is not of the same type no exception is thrown, nor when using a safe cast does it result in a null. Why is this?

data class Rectangle(val width: Int, val height: Int)
data class Circle(val radius: Int)

fun main(args: Array<String>) {
    val listOfRects: List<Rectangle> = listOf(Rectangle(5,5))

    val listOfUnkown: List<Any?> = listOfRects

    val listOfWrongType: List<Circle> = listOfUnkown as List<Circle>
    // also works, thought should throw null
    // val listOfWrongType: List<Circle>? = listOfUnkown as? List<Circle>

    print(listOfWrongType)
}

Output

Test.kt:9:44: warning: unchecked cast: List<Any?> to List<Circle>
    val listA: List<Circle> = listOfUnkown as List<Circle>
                                           ^
[Rectangle(width=5, height=5)]

I also tried making deep copy of the lists when I was setting them just as a sanity check.

Answer 1

In Kotlin, the information about actual type arguments of generic types is erased at runtime, and type arguments thus cannot be checked during casts.

That's exactly what an unchecked cast is and what the warning tells you about. You can do such a cast but you have to accept that there is a possible type mismatch at runtime by suppressing the warning.

See the language reference:

• Type erasure and generic type checks

• Unchecked casts

Related Q&A:

• How to work with List casts: Unchecked Cast: List<Any?> to List<Waypoint>